Recently an oxyfuel combustor has been reviewed as one of techniques for treating carbon dioxide (CO2) which is said to be one of factors for global warming, and attention has been attracted to, for example, a coal-fired boiler for oxyfuel combustion of pulverized coal. In such coal-fired boiler, oxygen is used as an oxidizing agent in lieu of air to produce exhaust gas mainly composed of carbon dioxide (CO2) and such exhaust gas with high CO2 concentration is compressed and cooled into liquefied carbon dioxide. It has been reviewed that such liquefied carbon dioxide is transported by carrying means such as a vessel or a vehicle to a destination for storage thereof in the ground or alternatively the liquefied carbon dioxide increased in pressure is transported through a pipeline to a destination for storage thereof in the ground.
The exhaust gas from the coal-fired boiler upon such oxyfuel combustion of coal contains, in addition to carbon dioxide (CO2), impurities derived from coal feedstock such as nitrogen oxides (NOx), sulfur oxides (SOx), hydrargyrum (Hg), hydrogen chloride (HCl) and dust.
Among the above-mentioned impurities, sulfur oxides (SOx) may be contacted with and dissolved in water into sulfuric acid (H2SO4) and hydrogen chloride (HCl) may be dissolved in water into hydrochloric acid, so that such water-soluble sulfur oxide and hydrogen chloride as well as dust may be separated through contact with water by means of water splay or the like.
Among the nitrogen oxides (NOx) as the above-mentioned impurities, nitrogen dioxide (NO2) may be contacted with and dissolved in water into nitric acid (HNO3) to become separated. However, the exhaust gas from the coal-fired boiler has less oxygen (O2) so that nitrogen (N2) exists substantially in the form of nitrogen monoxide (NO) which is water-insoluble and thus is unremovable by water spraying or the like.
Among the above-mentioned sulfuric acid, hydrochloric acid and nitric acid, specifically sulfuric acid is known to corrode instruments in the exhaust gas treatment device; and hydrargyrum, which is trace metal as mentioned in the above, is known to hurt low-temperatured aluminum members constituting a heat exchanger arranged for a carbon dioxide liquefier. Thus, it is preferable to remove these impurities in the exhaust gas at early stages. There is also a problem that admixture of the impurities into the exhaust gas lowers a purity degree of the carbon dioxide, which makes troublesome the liquefaction of the carbon dioxide through compression and cooling and thus requires larger-sized equipment for liquefaction. Thus, in a system such as a coal-fired boiler for oxyfuel combustion where exhaust gas mainly composed of carbon dioxide is produced and the carbon dioxide is disposed, it is extremely important to remove impurities in the exhaust gas.
Thus, it has been conducted, for example, in the coal-fired boiler for oxyfuel combustion that a spray-column-type, packed-column-type or other wet desulfurizer used in a conventional air-fired boiler or the like is provided to remove sulfur oxides which are especially problematic in corrosion. Moreover, nitrogen and nitrogen oxides derived from coal feedstock are produced in the exhaust gas from the coal-fired boiler for oxyfuel combustion or the like, so that it has been conducted that a catalyst-type or other denitrator is arranged upstream of the desulfurizer to remove the nitrogen and nitrogen oxides.
It is known in the wet desulfurizer provided as mentioned in the above that sulfur oxides and hydrogen chloride as well as dust are removed and that nitrogen oxides are partly removed and hydrargyrum, which is inherently low in content, is slightly removed. It has been also conceived that if hydrargyrum in the exhaust gas is still high in concentration even after the above-mentioned exhaust gas treatment is conducted, a hydrargyrum-removing column is arranged to remove the hydrargyrum by adsorbent or the like.
As mentioned in the above, the exhaust gas mainly composed of carbon dioxide (CO2) from the coal-fired boiler for oxyfuel combustion usually undergoes compression by a plurality of compressors, cooling by aftercoolers respectively downstream of the compressors and eventual liquefaction into liquefied carbon dioxide. However, in this case, there is a problem that the compressors may be corroded by sulfuric acid (H2SO4) resulting from sulfur oxides (SOx) included in the exhaust gas. Thus, it is a very important task to prevent the compressors, which are extremely expensive, form being corroded.
Patent Literature 1 discloses an exhaust gas treatment system comprising a duct with a dust collector and a wet desulfurizer to which exhaust gas is guided from a boiler which in turn burns fuel with mixed combustion gas of oxygen-rich gas with circulated exhaust gas, an exhaust gas recirculation duct for guidance of part of the exhaust gas downstream of the dust collector to the boiler and CO2 separation means for compression of the exhaust gas downstream of the desulfurizer to separate carbon dioxide. Water separated in the compression of the exhaust gas by the CO2 separation means is supplied to absorbing liquid circulatorily used in the desulfurizer.
Patent Literature 2 discloses a gas treatment installation in which a gas flow including non-absorbing gas such as a hydrocarbon gas or nitrogen is treated by co-current contactors arranged in series.
Patent Literature 3 discloses a gas treatment apparatus in which raw air is compressed and introduced into a catalyst column where an infinitesimal quantity of carbon monoxide and hydrogen are converted into carbon dioxide and water. Catalyst-reacted temperature-increased air is cooled and introduced into an adsorption column where carbon dioxide, water and other impurities are adsorption-removed to obtain high-purity product air, a remaining part being introduced into and cooled by a primary heat exchanger substantially down to a liquefaction temperature. The cooled remaining part is introduced into a simple rectification column and is subjected to liquefaction rectification to thereby obtain high-purity nitrogen and oxygen-enriched liquefied air.
Patent Literature 4 discloses an exhaust gas treatment apparatus comprising a dust remover for removal of dusts in exhaust gas, an absorbing column arranged downstream of the dust remover for absorptive removal of SOx, HCl and the like, an undercooling mist eliminator arranged downstream of the absorbing column for removal of dusts in an agglomerated bloating manner and a catalyst device for decomposition of harmful matters in the exhaust gas.
Patent Literature 5 discloses a flue gas treatment system for control of pH of an absorbent slurry comprising means for dosing of an alkaline agent into the absorbent slurry, a pH detector of the slurry, means for detection of an operational state of a gas-gas heater, means for detection of an operational state of a dust collector and means for controlling of the dosage of the alkaline agent on the basis of signals from the pH detector, the gas-gas heater and the means for detection of the operational state of the dust collector.